Discrete-time radio frequency (RF) is a newly emerging field in wireless digital communications wherein analog RF signals that are transmitted over-the-air are directly sampled into a discrete-time sample stream suitable for digital signal processing. A typical wireless digital communications device would use analog filters, duplexers, mixers, analog-to-digital converters (ADC), etc. to convert the analog RF signals into a digital data stream that is suitable for digital signal processing. Unfortunately, analog circuit components, especially components such as capacitors, inductors, resistors, etc. necessary for the analog filters are difficult to integrate into an integrated circuit. This is especially true for the precise values of these components required for use in filters. Of course, it is the desire of the manufacturer to maximize the degree of integration for the wireless transceivers. This is because the more highly integrated a wireless transceiver can become, the lower the production costs for the transceiver and the transceiver will typically use less power during operation.
Discrete-time RF involves the direct conversion of the analog RF signal into a discrete-time sample stream through the use of a direct sampling mixer, without having to undergo any intermediate analog filtering, downconversion, etc. An example of a direct sampling mixer is one that uses current to perform its sampling. The current-mode discrete-time sampling mixer converts the received analog RF signal into a current that is then integrated by a sampling capacitor. The charge on the sampling capacitor is then periodically read to produce samples for the discrete-time sample stream.
Many mixers work with more than one signal stream, i.e., the RF current is periodically integrated at various points in time to produce multiple sample streams. For example, it is fairly typical for a digital transceiver to process the received signal stream as two separate streams, an in-phase (I) stream and a quadrature-phase (Q) stream. Additionally, many use differential signaling, wherein a portion of each signal is received along with a portion of the same signal that is 180 degrees out of phase. Therefore, the mixers can be quite complex, with four separate signal paths.
A significant disadvantage of having four separate signal paths in the mixer stems from the fact that each signal path requires a different clock. For example, the clock for a positive I stream will differ from the clock for a positive Q stream by 90 degrees and the clock for a positive stream will differ from the clock for a negative stream by 180 degrees. Since the clocks typically differ from one another by a phase angle, it is common for each signal path will have its own clock generation hardware.
One possible solution to providing separate clocks to each signal path is to provide each signal path with its own local oscillator (LO) generating a signal of desired period with the proper timing and then a series of clock dividers to generate clocks of the proper frequency from the signal generated by the LO. However, the use of a different LO for each signal path can result in synchronization problems due to frequency differences in the signals generated by the different LOs, resulting in a degraded downconverted signal. Alternatively, there may be a single LO, whose signal is fed to each of the signal paths and each signal path has its own clock generating hardware that would take the signal from the LO and derive the necessary clock signals. Regardless of whether a single LO or multiple LOs are used, there is typically a separate set of clock generating hardware for each signal path.
A major disadvantage in having separate clock generating hardware for each signal path is power consumption. As expected, the clock generating hardware must also be clocked at high frequencies and hardware clocked at high frequencies consumes more power than hardware clocked at low frequencies. Also for more complex clocking schemes, a large amount of hardware is required for the clock generating hardware. The clocking at high operating frequencies and the redundancy of the generating hardware results in a significant amount of power consumption. For example, in a mixer with four signal paths, four complete sets of clock generating hardware are required. An additional disadvantage is that the redundant clock generating hardware also requires a lot of real estate when it comes time to integrate the mixer hardware into an integrated circuit. The increased real estate results in a larger, more expensive device.
A need has therefore arisen for a way to generate clock signals required in a mixer with multiple signal paths without needing to replicate the clock generating hardware for each signal path.